1. Field of the Invention
The present invention relates to the field of measurement, and in particular to an orthogonality compensation method for length measurement device and a length measurement device using the method.
2. The Related Arts
Liquid crystal display (LCD) has a variety of advantages, such as thin device body, low power consumption, and being free of radiation, and is thus widely used. Most of the LCDs that are currently available in the market are backlighting LCDs, which comprise a liquid crystal display panel and a backlight module. The operative principle of the liquid crystal panel is that liquid crystal molecules are interposed between two parallel glass substrates and the liquid crystal molecules are controlled to change direction by applying electricity to a circuit of the glass substrates in order to refract out light emitting from the backlight module for generating images.
Referring to FIG. 1, the liquid crystal display panel generally comprises: a TFT (Thin-Film Transistor) substrate 100, a CF (Color Filter) substrate 300 laminated on the TFT substrate 100, and liquid crystal 500 interposed between the TFT substrate 100 and the CF substrate 300. The TFT substrate 100 generally comprises a substrate 102 and a TFT array 104 formed on the substrate 102. The TFT array 104 is formed on the substrate 102 through mask processes. During the fabrication of the TFT array 104, to ensure dimension preciseness of each pixel unit, it is necessary to measure the dimensions of each pixel unit.
Referring to FIGS. 2 and 3, in laminating the TFT substrate 100 and the CF substrate 300, to ensure the accuracy of lamination, it is a common practice to arrange alignment marks on corners of the TFT substrate 100 and the CF substrate 300. Generally, first marks 120 that are cruciform are provided at four corners of the TFT substrate 100 and second marks 320 are arranged at four corresponding corners of the CF substrate 300. Each of the second marks 320 comprises four mark blocks 322. The four mark blocks 322 are symmetric to each other. When the TFT substrate 100 is laminated on the CF substrate 300, the second marks 320 are completely corresponding to the first marks 120. Under this condition, the four mark blocks 322 of each of the second marks 320 are respectively located at four corners of the first mark 120 that corresponds to the second mark 320. In the formation of the first and second marks 120, 320, necessary measurement must be carried out with a length measurement device in order to ensure the accuracy of matching. After the lamination of the TFT substrate 100 and the CF substrate 300, it also needs to carry out inspection with the length measurement device to ensure precise matching between the first and second marks 120, 320.
In summary, it is apparent that the length measurement device plays an important role in the manufacturing process of the liquid crystal display panel and is necessary equipment for the manufacturing of liquid crystal display panels.
Referring to FIG. 4, a conventionally used length measurement device is generally a laser length measurement device, which comprises: a measuring platform 500 and an operation system (not shown) electrically connected to the measuring platform 500. The measuring platform 500 comprises a chassis 502, a table 504 that is mounted on the chassis 502, guide rails 506 that are mounted at opposite sides of the table 504, a cross bar 508 that is slidably mounted on the guide rails 506, an inspection microscope 510 that is slidably mounted on the cross bar 508, and a laser device 512 that is mounted on the chassis 502 and coupled to the inspection microscope 510. To carry out measurement, the operation system controls the measuring platform 500 to read coordinates of two measurement points on an object to be measured (not shown) that is positioned on the table 504. The two sets of coordinates so read are transmitted to the operation system. The operation system performs computations according to the two sets of coordinates in order to determine the length between the measurement points, which will be specifically described as follows:
With reference to FIG. 5, in which X−Y is a measurement coordinate system, Xt−Yt is a coordinate system of the table, and Xc−Yc is a coordinate system of the inspection microscope, the coordinates of a measurement point A are (X, Y)=(Xt+Xc, Yt+Yc).
For a length measurement device that is used in a manufacturing process of liquid crystal display panel, the table 504 comprises a plurality of support pins 542 mounted thereon. A liquid crystal display panel (not shown) is positionable on the support pins 542. Lengthwise direction of the guide rails 506 is taken as the direction of Y-axis and the lengthwise direction of the cross bar 508 is taken as the X direction. Thus, the cross bar 508 must be perpendicular to the guide rails 506. However, in a process of actual measurement, when the cross bar 508 slides with respect to the guide rails 506, variation of orthogonality between the cross bar 508 and the guide rails 506 may occur (see FIG. 6), leading to errors of measurement and thus affecting the subsequent manufacturing operations.